The present invention relates to method for a vehicle with an electric motor, especially to a method for an electric vehicle or a hybrid vehicle, as well as to such a vehicle.
In a vehicle with an electric motor as the drive motor such as, for example, an electric vehicle or a hybrid vehicle, the behavior and operating conditions differ from those of a vehicle with an internal combustion engine as the drive engine. This especially pertains, for instance, to the operation at low vehicle speeds since an electric motor can fundamentally be operated at any desired low motor speeds, whereas an internal combustion engine can usually only be operated within a range above a predetermined idling speed. Another example is overrun operation, during which a different deceleration can occur in an electric vehicle due to recuperation. Since most car drivers are only used to the behavior of vehicles with internal combustion engines, the driver of an electric car or of a hybrid car might be confused by the different behavior, or else dangerous situations might even arise due to this different behavior. For this reason, the state of the art discloses various methods to make the behavior of a vehicle with an electric drive similar to the behavior of a vehicle with an internal combustion engine.
For example, German patent application DE 10 2007 055 785 A1 discloses a method for controlling the inching mode of operation of a vehicle with a hybrid drive so as to allow an efficient inching operation that is largely unlimited in time but that is nevertheless reliable, whereby an electric power storage unit is utilized as little as possible. For this purpose, a power train essentially comprises an internal combustion engine, an electric motor, a shifting element arranged between the internal combustion engine and the electric motor, a transmission and a driven element. When the internal combustion engine is running, primarily the shifting element is employed to effectuate the inching operation, while secondarily, the electric motor is employed as a function of the monitoring of prescribed operating parameters of the shifting element.
German patent specification DE 199 21 918 C2 relates to a parking lock for a vehicle with an electric drive. The parking lock comprises a mechanical and an electrical component. Upon actuation of the parking lock, the mechanical component is mechanically put in operative connection with an electric power train in order to stop its rotational movement. The electrical component is configured in such a way that, when the parking lock is about to be actuated, the electrical component can generate a short circuit, at least temporarily, in a stator winding of the electric motor.
German patent application DE 10 2008 053 505 A1 relates to a method for controlling a hybrid power train of a motor vehicle in order to achieve a smooth behavior of the wheel torque when the clutch is engaged and disengaged. The clutch is arranged in a hybrid power train and has a clutch input and output. On the clutch input side, the hybrid power train has an internal combustion engine while on the clutch output side, the hybrid power train has a transmission as well as at least one electric motor. Once a synchronous speed range has been reached between the clutch input and the clutch output, the clutch is abruptly engaged.
In vehicles with an electric motor as the drive motor, for example, electric cars or hybrid cars, for the sake of efficiency, the electric motor is frequently coupled to the driven axle directly, without a disconnect-type clutch. This means that a driver cannot physically separate the drive from the driven elements (axle and wheel) by means of a disconnect-type clutch, as is the case in a vehicle with an internal combustion engine. Nevertheless, even in vehicles with an electric drive motor, a gearshift lever is normally available to the driver which, similar to an automatic transmission, has at least the gearshift lever positions “D” for drive, “R” for reverse and “N” for neutral or for a “safe state” in which no torque is being transmitted from the drive motor to the wheels. Consequently, a driver who wants to select a torque-free state in the drive of the vehicle can do so in the usual way by setting the gearshift lever to “N”. This function in an electrically driven vehicle in combination with the fact that, in an electrically driven vehicle, braking torques are effectively applied onto the wheels by means of the electric motor via recuperation torques leads to the situation that, when the car is shifted from “D” or “R” to “N”, this braking torque has to be eliminated. The shift from “D” or “R” to “N” can occur, for instance, during maneuvering or during normal driving. If the vehicle is moving during the shifting or if the brake of the vehicle is simultaneously activated during the shifting, the driver experiences the elimination of the braking torque as if it were an acceleration since the overall deceleration effect is diminished. The driver can perceive this as unpleasant or confusing and this might pose a safety hazard in critical situations.